In the United States, cements are divided into the following categories: (1) Portland cement; (2) Natural cement; (3) High alumina cement; (4) Supersulphate cement; and (5) Special cements. This invention is generally related to an improved cementitious system for use in blended or masonry cements as a substitute for ordinary Portland cement.
To assist the reader in understanding the processes and compositions of this invention, a listing of terms and their basic definitions is set forth below, as well as a basic description of how ordinary Portland cement is prepared and tested. This information is not supplied as a limitation to the invention and should not be used as such. The scope and breadth of the invention is as set forth in the claims.
A. Definitions
Ordinary Portland cement is a hydraulic cement produced by pulverizing Portland cement clinker. Portland cements are classified under ASTM standards (C 150-94) into eight types, including:
Type I. For use in general concrete construction where the special properties specified for Types II, III, IV and V are not required.
Type II. For use in general concrete construction exposed to moderate sulphate action, or where moderate heat of hydration is required.
Type III. For use when high early strength is required.
Type IV. For use when low heat of hydration is required.
Type V. For use when high sulphate resistance is required.
Type IA, IIA and IIIA are the same as Types I, II, and III respectively except that they have an air entraining agent added. "Ordinary Portland cement" in the context of this patent covers all types (I-V and IA-IIIA) of Portland cement as referenced in ASTM C 150-94.
Cement clinker is the sintered product produced by the kiln system. In ordinary Portland cement, the clinker is generally a partially fused product consisting essentially of hydraulic calcium silicates.
Blended cement is generally a hydraulic cement comprising an intimate and uniform blend of ordinary Portland cement and pozzolanic materials produced by (1) intergrinding the ordinary Portland cement clinker with the pozzolanic materials; or (2) interblending ordinary Portland cement with the pozzolanic materials.
Masonry cement is a hydraulic cement for use in mortars for masonry construction. It contains one or more of the following materials: ordinary Portland cement, Portland blast-furnace slag cement, Portland-pozzolan cement, natural cement, slag cement or hydraulic limes. It also usually contains one or more materials such as hydrated lime, limestone, chalk, calcareous shell, talc, slag or clay.
Hydraulic cement is a cement that sets and hardens by chemical interaction with water and is capable of doing so under water.
A cementitious system is the total combined dry mixture of finely divided hydraulic and pozzolan materials for a concrete which reacts with water to form the binder in concrete.
Concrete is a construction material comprised of the cementitious system, water, admixtures, and aggregates.
Pozzolan is normally a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
Blended pozzolan is a pozzolan blended with other components. The components may be any of several types of material, including: gypsum, alkali salts, hydrated kiln dust, hydrated lime, flyash, plasticizing agents, etc.
Slag is the common term for the non-metallic product, consisting essentially of silicates and aluminosilicates of calcium and other bases, that is developed in a molten condition simultaneously with iron in a blast furnace.
To calcine or calcining a material is to alter the composition or physical state of the material by heating the material to drive off volatile matter without fusing.
Intergrinding is the process of grinding the cement clinker and other additives to the desired fineness in a grinding mill.
Interblending is the process of adding materials to the cement after the cement clinker has already been ground in the grinding mill.
Normal consistency (nc) is the amount of water required to prepare cementitious systems to a given consistency as defined by ASTM C 187-94.
Efflorescence is the mechanism by which available alkalies are transported to masonry mortar surfaces and precipitate out upon drying to form a powered material. The precipitate is typically a sodium carbonate or calcium carbonate composition.
The property of low alkali functionality is the equivalent performance of a cementitious system to the performance of a low alkali Portland cement when tested by ASTM C 227-94 test methods. FIG. 12A illustrates the general performance of a low alkali cementitious system when tested by ASTM C 227-94.
The property of alkali non-reactiveness is when the cementitious system expands less than about 0.06% under the testing procedure of ASTM C 227-94. FIG. 12A illustrates the performance of an alkali non-reactive cementitious system when tested by ASTM C 227-94.
The property of alkali resistance is when cementitious systems have less than a about 0.08% expansion under ASTM C 1260-94 using a highly reactive aggregate. Alkali resistant cementitious systems offer protection from alkali attack far beyond that provided by low alkali functionality cementitious systems because alkali resistant cementitious systems actually protect the aggregate from attack. FIG. 12B illustrates the performance of an alkali resistant cementitious system.
A highly reactive aggregate is an aggregate that results in an expansion of about 0.6% or more under ASTM C 1260-94 using Type I Portland Cement. Highly reactive aggregates commonly used for testing are known as "New Mexico Aggregates" and "Canadian Spratt Aggregates."
A. Current Day Preparation of Ordinary Portland Cement
Ordinary Portland cement is generally prepared as schematically set forth in FIG. 1. The raw materials, which are generally comprised of limestone, sand, clay and iron ore, are fed proportionally into a grinding mill. In the grinding mill, the raw materials are ground to the desired fineness. After being ground, the raw materials are fed into the rotary kiln system for calcining.
After the feed passes through the rotating kiln, it is "cement clinker" and is passed over a clinker cooler which provides air to cool the cement clinker. The cement clinker is then passed into a grinding mill wherein gypsum is interground with the cement clinker to provide the ordinary Portland cement.
After being interground with the desired proportion of gypsum, the Portland cement is moved to bulk storage. The cement is then distributed to the customer.
When preparing ordinary Portland cement under conventional theories, typical grinding mills are fed two components, cement clinker and gypsum (CaSO.sub.4 .2H.sub.2 O). In the grinding mill, each component absorbs energy proportional to the amount of each component in the mill. For example, if the feed is comprised of 94% clinker and 6% gypsum, the clinker would absorb 94% of the energy and the gypsum would absorb 6% of the energy. The surface area of each component after being ground by the grinding mill is a function of the energy absorbed and the grindability of the component absorbing the energy. As expected, gypsum is easier to grind than cement clinker. Consequently, since the cement clinker and the gypsum absorb equivalent energy, the gypsum will be ground finer, resulting in the gypsum having a higher surface area than the cement clinker. This is a desirable characteristic in ordinary Portland cement because gypsum acts as a retarder. As a retarder, it must be quickly soluble in water. Due to its high surface area after intergrinding, gypsum is highly soluble.
Conventional theory teaches to operate grinding mills to exploit this difference in surface area. This conventional method of exploiting the surface area difference between the cement clinker and the gypsum, or any other material that is interground, will be termed herein "differential grinding."
C. Test Methods
Various ASTM test methods are used in determining and quantifying the desirable and undesirable qualities of cementitious systems prepared from Portland and blended cements. Some of these test methods include: (1) ASTM C 227-94, which quantifies the effects of internal alkalies and can be used to determine if cementitious systems have the properties of low alkali functionality or alkali non-reactiveness; (2) ASTM C 1260-94, which can quantify the effects of external alkalies and determines if a cementitious system is alkali resistant; (3) ASTM C 109-94 quantifies the compressive strength of a cementitious system; and (4) ASTM C 1202-92 measures the permeability of the cementitious system to chloride ions. ASTM test methods and standards including ASTM C 227-94, C 1260-94, C 109-94, C 1202-94, C 150-94, C 1157-94, C 595-94, C 1012-94 and AASHTO T 277-94 and all other test methods or standards referenced herein are hereby incorporated by reference as if set forth in their entirety. The -94 following the ASTM test method number indicates that it is the ASTM method in effect during 1994.
Although the ASTM test methods are set out specifically, those skilled in the art may be aware of alternative methods which could be used to test for the referenced qualities or results. The only difference being, the results or qualities may be reported in a different manner wherein a conversion system could be used to give comparable results. Consequently, the invention should not be limited by the referenced test methods and the results thereof, but rather only to the claims as set forth below taking into account equivalent testing methods and results.
i. Effects on Concrete by Internal Alkalies
Aggregates used in concrete mixtures contain mineralogical components that will react with hydroxyl ions in the concrete pore solution and form silica hydroxide gels. These silica hydroxide gels absorb the alkali ions producing alkali-silica gels in the concrete matrix. The alkali-silica gels are capable of absorbing water which causes the gels to swell in the confined spaces of the hardened concrete. The swelling creates internal stresses which result in premature cracking of the concrete. The above described reaction of silica hydroxide gels ultimately absorbing H.sub.2 O is termed "Alkali Silica Reactivity" (ASR).
ASR is a significant factor in the deterioration of concrete. Current teachings suggest that fewer alkali ions in the cement will decrease the occurrence of ASR. As a result, the cement specified for concrete that may experience ASR is currently limited to low alkali cement (less than about 0.40% to about 0.60% Na.sub.2 O equivalent). To manufacture a low alkali cement, either uniquely low alkali raw materials must be utilized, which is usually uneconomical, or the Portland cement is processed in such a manner that the naturally occurring alkalies are evaporated and become concentrated in a byproduct stream known as cement kiln dust (CKD).
As shown in FIG. 1, when the raw materials are being processed in the kiln system, the high alkali CKD evolves and is removed and transported to landfills as waste materials. In some systems, the amount of CKD removed amounts to as much as 15% of the total input of raw materials. Thus, a kiln system capable of producing a million tons of cement clinker a year could produce 150,000 tons or more of high alkali CKD.
Although low levels of alkali are already required in some instances, lower limits of alkali content are being proposed by both state and federal highway departments in hopes of further reducing ASR. Using the current method of producing Portland cement, lower levels will translate into additional CKD being removed and discarded, directly resulting in higher fuel and raw material consumption, and increased expense for CKD removal, while possibly not solving the ASR problem if the alkali attack is from external sources such as deicing salts.
Additionally, the Environmental Protection Agency (EPA) is considering establishing substantial controls on the disposal of CKD, possibly classifying it as a hazardous waste which would be even more expensive for the cement producer to discard.
Consequently, a need exists for a method and/or composition which eliminates the need to remove some or all of the CKD from the kiln system, thus considerably reducing the cement producers' cost of production and addressing other environmental concerns related to the disposal of the CKD, while simultaneously solving the problem of ASR due to internal alkalies.
ASTMC 227-94 is utilized to determine the susceptibility of cementitious system/aggregate combinations to undergo ASR by measuring the increase or decrease in length of mortar bars prepared from the cementitious system/aggregate combination. The aggregate utilized in ASTM C 227-94 can be either the job aggregate or a very reactive reference aggregate such as pyrex glass.
By comparing the results of ASTM C 227-94 tests on cementitious systems to those of low alkali Portland cements, it can be determined whether the cementitious system has the property of low alkali functionality. If the cementitious system performs similar to a low alkali Portland cement in C 227-94, it is classified as having the property of low alkali functionality. FIG. 12A illustrates generally how a Type 1 low alkali Portland cement performs under the conditions of ASTM C 227-94.
If the expansion in ASTM C 227-94 is less than about 0.06%, then the cementitious system not only has the property of low alkali functionality, but is also alkali non-reactive. FIG. 11A illustrates the cementitious system of this invention, "Type 1P", which is alkali non-reactive as the expansion is less than about 0.06%. FIG. 12A also illustrates an alkali non-reactive cementitious system.
ii. Effects on Concrete by External Alkalies
External alkalies are such things as deicing salts, fertilizers or other chemicals placed on the lawn or ground next to the concrete, etc. External alkalies, like internal alkalies, can cause ASR expansion. Consequently, a need exists for a cementitious system that mitigates or at least minimizes ASR reactions due to external alkalies. A cementitious system that has these capabilities is termed an alkali resistant cementitious system.
ASTM C 1260-94 can be used to determine whether a cementitious system is resistant to external alkalies, and thus alkali resistant. Originally, ASTM C 1260-94 was developed to measure the susceptibility of aggregates, not the cementitious system, to alkali attack. In fact, C 1260-94 was originally thought to be independent of the type of cementitious system used. It has been found, however, that the cementitious systems of this invention can actually prevent the alkali from reacting with a highly reactive aggregate, such as a New Mexico aggregate, even under the very severe C 1260-94 test conditions. (See FIG. 11B.)
ASTM C 1260-94 simulates external alkalies by soaking a mortar bar specimen in a hot alkali solution. ASTM C 1260-94 measures the increase or decrease in length of mortar bar specimen to quantify the effects of the alkali on the mortar bar specimen. If the mortar bar specimen increases in size, ASR, as a result of external alkalies, has occurred, and therefore, external alkalies are adversely effecting the cementitious system. Meaning, the cementitious system is not alkali resistant. Comparatively, if the mortar bar specimen has an expansion of less than about 0.08%, the cementitious system is alkali resistant. Alkali resistant cementitious systems offer protection from external alkalies far beyond that provided even by low alkali cementitious systems. This is clearly illustrated in FIG. 11B. "Type 1" in FIG. 11B is the performance of a low alkali cementitious system under ASTM C 1260-94. "Type 1P" is the performance of a cementitious system of this invention which is alkali resistant.
iii. Compressive Strengths
ASTM C 109-94 measures the compressive strength of hydraulic cement mortars. The compressive strength is the measured maximum resistance of a mortar specimen to axial compressive loading normally expressed as force per unit cross-sectional area. In prior art mortars, which included calcined clays, the early compressive strengths during the first 7 days, and most markedly in the first day, are highly diminished.
The diminished strength is undesirable for several reasons. Initially, delay in early strength development results in surface cracking due to evaporation. Secondly, jobs take longer because the concrete form must remain in place substantially longer, and finishing is delayed.
Yet, cements containing calcined clays are desirable due to their enhanced long-term compressive strengths. Consequently, a need exists for a composition which incorporates calcined clay due to its beneficial attributes such as enhanced long-term compressive strengths, yet does not have the undesirable decreased early compressive strengths shown by prior art concretes containing pozzolanic materials.
iv. Chloride Permeability
AASHTO T 277-94 or ASTM C 1202-94 determines the electrical conductance of concrete to provide a rapid indication of its resistance to the penetration of chloride ions. The greater the chloride ion permeability, the greater the chance that the reinforcing steel will corrode and weaken. Consequently, a need exists for a composition with low chloride ion permeability such that the steel reinforcing materials do not corrode.
v. Water Requirement
ASTM C 187-94 measures the amount of water required for mixing with a cementitious system to obtain a desired consistency. In prior art cementitious systems which contained calcined clays, the clays caused an increase in water demand over the water demand of Ordinary Portland cement. The increased water demand was directly correlated to dramatic decreases in early compressive strengths of the prior art cementitious systems containing calcined clays with respect to Ordinary Portland cement. Consequently, a need exists for a cementitious system containing calcined clays which has a lower water demand and increased early compressive strength over that of prior art cementitious systems containing calcined clay.